RFID tag having multiple transceivers

ABSTRACT

Information is communicated between an RFID tag and first and second readers. A first transceiver of the RFID tag is controlled so that the first transceiver communicates with the first reader and so that the first transceiver has substantially longer periods during which the first transceiver is not in communication with the first reader than when the first transceiver is in communication with the first reader. A second transceiver of the RFID tag is controlled so that the second transceiver communicates with the second reader at least during the periods when the first transceiver is not in communication with the first reader. The RFID tag may also have a battery, a switch coupling the battery to at least the first transceiver, and a controller that operates the switch in a duty cycle such that power is provided by the battery to the first transceiver during ON times of the duty cycle and such that power from the battery to the first transceiver is interrupted during OFF times of the duty cycle.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a tag that can be suitably attached toan article and that can be used for RF communications with a tag reader.

BACKGROUND OF THE INVENTION

Various labels have been attached to articles so that the articles canbe distinguished one from the other. For example, bar code labels areattached to articles of grocery and are scanned at a check-out counterin order to automatically identify the articles and to register theprice of the articles as they are purchased.

Bar code labels have also been used in inventory control and monitoring.Accordingly, these bar codes may be scanned in order to track articlesas they move into, through, and out of a storage area. It is also knownto read the bar codes attached to articles in order to access variouscomputer records regarding the articles.

Bar code labels, however, have several drawbacks. For example, computerstored records that are accessed when a bar code is read do not movewith the corresponding article. Therefore, if the article to which thebar code label is attached is remote from the computer, the recordsconcerning that article cannot be immediately accessed if necessary.

Moreover, bar code labels cannot be read remotely. Thus, if it isdesired to take an inventory of articles currently in the storage area,personnel must physically scan each label on each article one at a timein order to determine which articles are presently in the storage area.Such scanning requires the physical presence of the personnel at thelocation of the articles and is extremely time consuming. Additionally,because bar code labels cannot be read remotely, they cannot be used assecurity devices that can be detected if the articles to which they areattached are improperly removed from a secured area.

Instead of bar coded labels, it is known to attach radio frequencyidentification (RFID) tags to the articles to be monitored. The RFIDtags can be read, as can bar code labels. However, unlike bar codelabels, reading RFID tags does not require the physical presence ofpersonnel because the RFID tags can instead be read remotely. Thus,inventory can be taken more quickly because personnel are not requiredto walk around a storage area or other area in order to read the RFIDtags. Moreover, because RFID tags can be read remotely, they can be usedas security devices. Thus,if someone attempts to surreptitiously removean article to which an RFID tag is attached from a secured area, aremote reader can sense the RFID tag and provide an appropriate alarm.

RFID tags can be read one at a time or in groups. When multiple RFIDtags in a group are read at the same time, the information transmittedby the multiple tags frequently collide. Accordingly, spread spectrumtechniques, such as either direct sequence spread spectrum (DSSS) orfrequency hopping, in the communications between the reader and the tagshave been suggested in order to reduce the impact of such collisions. Itis also known to interrogate a tag using either a direct sequence spreadspectrum (DSSS) signal or a frequency hopping signal.

An RFID tag requires a power source in order to permit the transmissionof information from the tag to a reader. Traditionally, an RFID tag ispowered either locally or remotely. In remote powering of an RFID tag,the RFID tag typically derives its power from the signal transmitted bythe reader. A capacitor or other similar storage device stores the powerand supplies the stored power to the processing, memory, and transceiverof the RFID tag. One disadvantage of this powering technique is that, ifthe RFID tag is operated too far away from the reader, the RFID tagcannot derive sufficient energy from the reader's signal to effectivelypower its components.

Local powering of the RFID tag usually involves using a battery on theRFID tag in order to power the processing, memory, and transceiver.While the use of a local battery overcomes the disadvantage of operatingthe RFID tag too far away from the reader when the RFID tag is derivingits power from the reader's signal, the use of a local battery has thedisadvantage that it requires frequent replacement or re-charging due tothe amount of power consumed by the processing, memory, and transceiverof the RFID tag.

It is known to duty cycle a portion of a receiver of a tag in order toconserve battery power. An example of such a receiver is asuper-regenerative receiver. The receiver includes an amplificationstage, a local oscillator, a quench frequency source, and a detectorstage. The tag also includes a microcontroller, an RF transmitter, andan antenna. The tag remains in a low-power quiescent stand-by stateuntil it is activated by a signal from the reader. Followingtransmission of an activation signal by the reader, the reader sends arequest for information. Specifically, the receiver is duty cycled inorder to provide quiescent operation with a low current draw. Thus, someelements of the receiver are completely shut down to save tag powerwhile the tag is operating in a quiescent state. Amplifiers of thereceiver utilize forward-biased transistor stages whose forward biasingis provided at the duty cycle rate such that power draw is limited dueto the duty cycle of the bias. A duty cycle of 1 to 5% is thought toprovide sufficient time for reception of the activation signal and toreduce total current draw.

However, such an arrangement has a number of problems. For example,power consuming elements of the tag and even of the receiver may not beshut down during the OFF times of the duty cycle. Accordingly, power isstill wasted. Also, if the transmitter of the tag is capable of usingseveral frequencies specified by the reader, the tag described abovecannot quickly acquire the desired frequency for its transmitter andmust remain on for a sufficient period of time in order to permitacquisition of that frequency.

Moreover, when a tag is turned off and on in order to conserve batterypower, or when the tag is infrequently interrogated by a tag reader, orotherwise, it has periods of OFF time during which it is not or cannotbe interrogated by a tag reader. Therefore, interrogations by the tagreader can occur only at widely separated interrogation intervals,thereby allowing an article to which the tag is attached to be removedfollowing one interrogation without anyone realizing it until asucceeding interrogation.

The present invention overcomes one or more of these or other problems.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method ofcommunicating information between an RFID tag and first and secondreaders comprises the following: controlling a first transceiver of theRFID tag so that the first transceiver communicates with the firstreader and so that the first transceiver has substantially longerperiods during which the first transceiver is not in communication withthe first reader than when the first transceiver is in communicationwith the first reader; and, controlling a second transceiver of the RFIDtag so that the second transceiver communicates with the second readerat least during the periods when the first transceiver is not incommunication with the first reader.

In accordance with another aspect of the present invention, an RFID tagcomprises first and second transceivers. The first transceiver transmitsand receives first signals to and from a first reader. The secondtransceiver transmits and receives second signals to and from a secondreader.

In accordance with still another aspect of the present invention, amethod is providing to conserve battery power in an RFID tag having abattery, a receiver, and a transmitter. The method comprises thefollowing: duty cycling the receiver so that the receiver is turned onduring ON times of duty cycles and so that the receiver is turned offduring OFF times of the duty cycles; during the ON times of thereceiver, receiving a frequency from a tag reader; and, transmittingdata to the reader at the frequency.

In accordance with yet another aspect of the present invention, an RFIDtag comprises a transmitter, a receiver, a battery, a switch, and acontroller. The transmitter transmits first data to a tag reader. Thereceiver receives second data from the tag reader. The switch couplesthe battery to the receiver. The controller operates the switch in aduty cycle such that power is provided by the battery to the receiverduring ON times of the duty cycle and such that power from the batteryto the receiver is interrupted during OFF times of the duty cycle.

In accordance with a further aspect of the present invention, an RFIDtag comprises a transceiver and a receiver. The transceiver transmitsand receives first signals to and from a first reader. The receiverreceives second signals from a second reader and activates thetransceiver thereby causing the transceiver to transmit and receive thefirst signals to and from the first reader.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will become more apparent from adetailed consideration of the invention when taken in conjunction withthe drawings in which:

FIG. 1 illustrates a tagging system in accordance with one embodiment ofthe present invention;

FIG. 2 illustrates additional detail of a tag that can be used with thetagging system of FIG. 1;

FIG. 3 illustrates additional detail of a long range reader that can beused with the tagging system of FIG. 1;

FIG. 4 illustrates a message format useful in supporting communicationsbetween the tag and the reader of FIG. 1;

FIG. 5 illustrates an exemplary composition of a frame of the messageformat shown in FIG. 4;

FIG. 6 illustrates an exemplary composition of the header of the frameshown in FIG. 5;

FIG. 7 illustrates an exemplary composition of a time slot of the frameshown in FIG. 5;

FIG. 8 illustrates an exemplary composition of the header of the timeslot shown in FIG. 6; and,

FIGS. 9, 10, and 11 are flow charts showing an exemplary operation ofthe tag illustrated in FIGS. 1 and 2.

DETAILED DESCRIPTION

As illustrated in FIG. 1, a tagging system 10 includes a long rangereader 12, a short range reader 14, and an RFID tag 16. The long rangereader 12 includes an antenna 18, and the RFID tag 16 similarly includesan antenna 20. The antennas 18 and 20 establish a long range RF linkbetween the long range reader 12 and the RFID tag 16 so that the longrange reader 12 can remotely read the identification stored in a memoryof the RFID tag 16. The range of the long range reader 12 can be as highas several hundred feet or more. For example, the long range reader 12can have an expected range of approximately 500 feet.

A short range link 22 between the short range reader 14 and the RFID tag16 permits the short range reader 14 to read information from the RFIDtag 16 over a shorter range and/or in a more secure manner. For example,it may not be desirable for the long range reader 12 to read certaininformation stored in the RFID tag 16 because long range RFcommunications can be intercepted by a strategically placedsurreptitious reader similar to the long range reader 12. Accordingly,the short range link 22 increases the difficulty in illicitly acquiringthe more sensitive information that may be stored on the RFID tag 16.

The short range link 22 is shown in FIG. 1 as a hard wire link betweenthe short range reader 14 and the RFID tag 16. Accordingly, the moresensitive information stored on the RFID tag 16 can be read byestablishing a physical interconnection between the short range reader14 and the RFID tag 16.

Alternatively, the short range link 22 may be a limited range magneticlink such as those provided by contact-free smart cards. As a stillfurther alternative, the short range link 22 may be a very limited rangeRF link. Other alternatives will occur to those skilled in the art. Oneadvantage of using one of these non-hardwired alternatives for the shortrange link 22 is that then the RFID tag 16 can be more readily used as asecurity device. Accordingly, when an attempt is made to remove anarticle to which the RFID tag 16 is attached, the short range reader 14located at a portal of a secured area or otherwise can pick up a signalfrom the RFID tag 16 indicating that an attempt is being made to removethe article from the secured area.

The expected maximum range of the short range reader 14 over the shortrange link 22, for example, may be less than four feet, and is expected,in typical usage, to be between six inches and eighteen inches.

An embodiment of the RFID tag 16 is shown in additional detail in FIG.2. The RFID tag 16 includes a first transceiver 30 comprising afrequency agile (frequency hopping) RF transmitter 32 and a directsequence spread spectrum RF receiver 34. The frequency agile RFtransmitter 32 and the direct sequence spread spectrum RF receiver 34are coupled between the antenna 20 and a microprocessor 36. Accordingly,the frequency agile RF transmitter 32 of the RFID tag 16 implementsfrequency hopping in transmitting information to the long range reader12, and the direct sequence spread spectrum RF receiver 34 of the RFIDtag 16 implements direct sequence spread spectrum synchronization anddecoding in receiving communications from the long range reader 12.

The RFID tag 16 also includes a second transceiver 38 between themicroprocessor 36 and the short range reader 14. Accordingly, the RFIDtag 16 can transmit and/or receive communications to and/or from theshort range reader 14. In the case where the short range link 22 is ahardwire link, the second transceiver 38 may simply be a plug that isconnectible to a corresponding plug of the short range reader 14. In thecase where the short range link 22 is an RF link, the second transceiver38 may be an RF transceiver of any known type provided that this RFtransceiver preferably has a much shorter range than the frequency agileRF transmitter 32 and the direct sequence spread spectrum RF receiver34. In the case where the short range link 22 is a magnetic link, thesecond transceiver 38 may simply be a magnetic emitter (and/or sensor)capable of magnetically interfacing with the short range reader 14.

The RFID tag 16 further comprises a memory 40 coupled to themicroprocessor 36. The memory 40 stores the ID of the RFID tag 16 thatcan be read by the long range reader 12 through the antennas 18 and 20,the frequency agile RF transmitter 32, the direct sequence spreadspectrum RF receiver 34, and the microprocessor 36. The memory 40 mayalso store information supplied to it by the short range reader 14through the short range link 22, the second transceiver 38, and themicroprocessor 36. The memory 40 can additionally store informationsupplied by the long range reader 12.

This information can include, for example, the inventory history of thearticle to which the RFID tag 16 is attached. Accordingly, the date thatthe article entered inventory, the date that the article left inventory,the length of time that the article has been in inventory, any movementwithin inventory, and similar information may be stored in the memory40.

The information stored in the memory 40 may also include shippingmanifests that indicate when and to whom the article is to be shipped.Moreover, in the case where individual articles with differingdestinations are shipped in the same container, an RFID tag attached tothe container, hereafter called a container tag, can be attached to thecontainer. This container tag may be arranged to store the identity anddestination of each article in the container. As articles are removedfrom the container, the information stored in the container tag can beupdated to indicate which articles have been removed, the location atwhich the articles were removed, and the identity of the personnel whoremoved the articles.

The information stored in the memory 40 may further include maintenance,repair, and date of service records showing the maintenance and/orrepair history of the corresponding article.

Other information related to the article may likewise be stored in thememory 40. For example, the integrity of the information stored in thememory 40 can be assured by keeping a record of the modifications to thestored information and of the identity of the personnel making themodifications. As another example, records related to the production ofthe article may be stored in the memory of the tag.

Accordingly, any information about the article may be stored with thearticle instead of in a remote computer system or on paper.

Because the records are carried by the RFID tag 16 attached to acorresponding article, the RFID tag 16 eliminates the need to maintainpaper or computer records of the life history of an article, the RFIDtag 16 eliminates the problem of lost or misplaced records, and the RFIDtag 16 improves operational efficiency by eliminating the requirement toretrieve records prior to accessing and/or operating on the article.

The RFID tag 16 includes a battery 42 that is coupled so that itsupplies power through a switch 44 to the frequency agile RF transmitter32, through a switch 46 to the direct sequence spread spectrum RFreceiver 34, directly to the microprocessor 36, directly to the secondtransceiver 38 (if necessary), and directly to the memory 40.

Moreover, a plurality of sensors (not shown) may be coupled to themicroprocessor 36. These sensors may include, for example, a temperaturesensor, a humidity sensor, and other sensors such as a pressure sensor,a proximity sensor, an electromagnetic sensor, an optical sensor, amechanical sensor, a chemical sensor, and/or the like. Themicroprocessor 36 stores the information from the sensors in the memory40, and this information may be read from the memory 40 by the shortrange reader 14 or by the long range reader 12.

The microprocessor 36 may be arranged to further sense the voltage levelof the battery 42. Accordingly, the microprocessor 36 stores thisvoltage level in the memory 40, and this stored voltage level may beread from the memory 40 by the short range reader 14 or by the longrange reader 12. Thus, if the voltage level of the battery 42 as read byeither the short range reader 14 or the long range reader 12 indicatesthat the battery 42 needs charging or replacement, suitable remedialaction may be taken.

Because of the frequency agile RF transmitter 32, the direct sequencespread spectrum RF receiver 34, and/or the battery 42, the RFID tag 16is capable of relatively long range activation while providing a lowpower method for command-response activation by the long range reader12. This long range activation allows the RFID tag 16 to be placed atdistances remote from the long range reader 12 for purposes ofinterrogating the RFID tag 16 for its unique tag number and possiblyother information.

The frequency agile RF transmitter 32 and the direct sequence spreadspectrum RF receiver 34 allow the tagging system 10 to operate in theFCC defined Industrial Scientific and Medical (ISM) bands at maximumlegal power. Both frequency hopping as used by the frequency agile RFtransmitter 32 and direct sequence spread spectrum communications asused by the direct sequence spread spectrum RF receiver 34 circumventjamming by narrow-band signals using different methods of spreading thesignal over a large bandwidth. The direct sequence spread spectrum RFreceiver 34 can receive signals from the long range reader 12 withinmilliseconds of activation. By contrast, a frequency agile receiver mustsearch a long frequency hopping sequence in order to receive signalsfrom the long range reader 12. The time required to make this search istypically longer than the time required to detect a direct spreadspectrum sequence because the direct spread spectrum signal is either ona fixed frequency or on one of only a few frequencies.

Also, the RFID tag 16 may be duty cycled in order to conserve the energystored in the battery 42. Thus, each duty cycle has an ON time and anOFF time. During the ON time, the RFID tag 16 is fully poweredpermitting the direct sequence spread spectrum RF receiver 34 to receivecommunications directed to the RFID tag 16 and to permit the frequencyagile RF transmitter 32 to transmit information to in response to thecommunications. During the OFF time, all non-essential power consumingdevices of the RFID tag 16, such as the frequency agile RF transmitter32 and/or the direct sequence spread spectrum RF receiver 34, areeffectively disconnected from the battery 42 so that energy drainage ofthe battery 42 is reduced. The OFF time to ON time ratio of the dutycycle, for example, may be on the order of 1000:1, although other ratioscould be used.

An embodiment of the long range reader 12 is shown in additional detailin FIG. 3. The long range reader 12 includes a direct sequence spreadspectrum RF transmitter 50 and a frequency agile RF receiver 52 coupledbetween the antenna 18 and a microprocessor 54. The frequency agile RFreceiver 52 of the long range reader 12 implements frequency hopping inreceiving information from the frequency agile RF transmitter 32 of theRFID tag 16. Moreover, the direct sequence spread spectrum transmitter50 of the long range reader 12 implements direct sequence spreadspectrum transmission in transmitting communications to the directsequence spread spectrum RF receiver 34 of the RFID tag 16.

The long range reader 12 further comprises a memory 56 coupled to themicroprocessor 36. The memory 56 stores the information that the longrange reader 12 receives from the RFID tag 16. The memory 56 also storesthe software that supports a communication protocol as described herein.

This communication protocol governs the message format that is usedbetween the long range reader 12 and the RFID tag 16. According to thisprotocol, a message is comprised of a plurality of frames as shown inFIG. 4. Each frame is preferably no longer than the length of time thefrequency agile RF transmitter 32 is allowed to dwell at any givenfrequency.

Each of the frames shown in FIG. 4 has the construction shown in FIG. 5.Accordingly, each frame has a frame header and a number of time slotsTS0-TSN.

The frame header contains information about the long range reader 12that is reading the RFID tag 16. As shown in FIG. 6, the header contains(i) the state of the long range reader 12, (ii) the hop sequencecurrently being used by the long range reader 12 to receive messagesfrom the RFID tag 16, and (iii) the current position (i.e., frequency)of the long range reader 12 in this hop sequence. The frame header canalso contain such other information as may be useful in the taggingsystem 10. For example, the frame header may also contain the number(N+1) of the time slots in the corresponding frame.

The long range reader 12 may have several reader states including, forexample, an active communication state and a beacon state. In the activecommunication state, the long range reader 12 commands responses fromone or more selected tags such as the RFID tag 16. In the beacon state,the tags, such as the RFID tag 16, self-initiate the transmission ofmessages to the long range reader 12

The hop sequence and/or the current position in the hop sequence ascontained in the frame header are/is useful to tags that have limitedsignal processing capability. Such tags, for example, may have nocapability themselves to determine the frequency (i.e., the currentposition in the hop sequence) onto which they should transmit theirresponses.

Moreover, each time slot may also include a time slot header and data asshown in FIG. 7, and each time slot header, as shown in FIG. 8, maycontain the hop sequence and the current position in the hop sequence ofthe long range reader 12. The time slot header may also contain therelative position, such as a time slot number (0, 1, . . . , or N), ofthe corresponding time slot in the frame. This relative positioninformation may be used by the RFID tag 16 to establish a relativetiming interval into which the RFID tag 16 can transmit data. Bytransmitting the hop sequence and the current position in the hopsequence at the beginning of each time slot, the RFID tag 16 is aided inits rapid acquisition of the current hop sequence and frequency. Becausethe RFID tag 16 can acquire, from the header in each time slot,sufficient information about the frequency and timing of the long rangereader 12, the RFID tag 16 may power down until such time that itexpects the complete header information to be transmitted by the longrange reader 12. Therefore, the RFID tag 16 is able to substantiallyreduce the amount of power that it uses to determine the frequency andtiming to be used by its frequency agile RF transmitter 32 intransmitting information in the data portion of the time slot.

As indicated above, the long range reader 12 transmits all headers,whether frame headers or time slot headers. The RFID tag 16 transitsonly in the data portion of the time slots. The RFID tag 16 mayimplement a non-deterministic method of selecting a time slot for thetransmission of data. By using a non-deterministic method of selecting atime slot, the possibility of a plurality of tags transmitting data intothe same time slot is minimized. For purposes of illustration, such anon-deterministic method of selecting a time slot could be embodied by apseudo-random number generator that pseudo-randomly generates a numberof a time slot into which its corresponding tag transmits its data. Thisimplementation results in a communications protocol similar to, but notidentical to, the Aloha protocol, a standard communications protocol.

The long range reader 12 can communicate directly with a specific tag ora group of specific tags. When the long range reader 12 is communicatingdirectly with a specific tag or a group of specific tags, the long rangereader 12 may suspend the transmission of time slot headers. Thissuspension indicates to all other tags that their communications are tobe suspended. Also, all data may be transmitted between the long rangereader 12 and the RFID tag 16 in packets having packet numbers so thatboth the long range reader 12 and the RFID tag 16 can detect missing orduplicate data. Moreover, acknowledgements can be used to signify asuccessful transmission between the long range reader 12 and the RFIDtag 16. A failure to receive an acknowledgement can causere-transmission of the information. Once a transaction between the longrange reader 12 and a specific tag or group of tags is complete, thelong range reader 12 resumes transmitting the headers.

As discussed above, the RFID tag 16 may be duty cycled such that, duringON times, the RFID tag 16 can receive interrogations from the long rangereader 12 and such that, during OFF times, the RFID tag 16 cannotreceive interrogations from the long range reader 12. Alternatively, thelong range reader 12 may infrequently interrogate the RFID tag 16. Thus,during the OFF portions of the duty cycles, and/or during the longperiods between interrogations by the long range reader 12, an articleto which the RFID tag 16 is attached can be removed withoutauthorization from a secured area, and the long range reader 12 may notdetect this unauthorized removal of the article in sufficient time tostop it.

However, the presence of the second transceiver 38 enables the RFID tag16 to be immediately activated when the RFID tag 16 is brought into theshort range of the short range reader 14. The short range reader 14, forexample, may be located at an appropriate activation portal (typicallyinstalled at an entryway). In this case, the short range reader 14 canbe arranged to emit an interrogation signal that is read by the RFID tag16 through the second transceiver 38. The RFID tag 16 sends a responseto the interrogation signal through the second transceiver 38 in orderto indicate that the RFID tag 16 is within the range of the short rangereader 14. Therefore, an appropriate alarm can be given in sufficienttime to prevent the unauthorized removal of the article.

Alternatively, a portal may be arranged to interrogate the RFID tag 16and the RFID tag may be arranged to transmit a response through thesecond transceiver 38 to the short range reader 14 that is locatedelsewhere than at the portal. As a further alternative, a portal may bearranged to interrogate the RFID tag 16 and the RFID tag 16 may bearranged to wake up the first transceiver 30 causing the firsttransceiver 30 to transmit a response through to the long range reader12 that is located remotely from the portal. As a still furtheralternative, the portal and/or the short range reader 14 may passivelyreceive a signal from the second transceiver 38 of the RFID tag 16 inorder to indicate that the RFID tag 16 is within the range of the portaland/or the short range reader 14.

As shown in FIG. 9, when it is time for the RFID tag 16 to receivemessages from the long range reader 12, as indicated by a block 100, theRFID tag 16 powers up, as indicated by a block 102. Accordingly, poweris supplied to the direct sequence spread spectrum RF receiver 34 sothat it can then listen for a message from the long range reader 12. Topower up the direct sequence spread spectrum RF receiver 34, themicroprocessor 36 may simply close the switch 46 to couple the battery42 to the direct sequence spread spectrum RF receiver 34. Themicroprocessor 36 controls the duty cycle of the switch 46 so that ithas an appropriate OFF to ON ratio, such as the 1000:1 duty cycle ratiodescribed above.

When the RFID tag 16 detects a received message at the direct sequencespread spectrum RF receiver 34 as indicated by a block 104, the RFID tag16 parses the header information as indicated by a block 106 and, asindicated by a block 108, stores the reader state, the hop sequence, thecurrent position in the hop sequence, and any other information that iscontained in the header.

As shown in FIG. 10, when it is time for the RFID tag 16 to transmitdata to the long range reader 12 as indicated by a block 110, the RFIDtag 16 powers up, as indicated by a block 112. Accordingly, power issupplied to the frequency agile RF transmitter 32 so that it can thentransmit data to the long range reader 12. To power up the frequencyagile RF transmitter 32, the microprocessor 36 may close the switch 44to couple the battery 42 to the frequency agile RF transmitter 32. Themicroprocessor 36 controls the duty cycle of the switch 44 so that ithas an appropriate OFF to ON ratio such as the 1000:1 ratio describedabove. Thereafter, the RFID tag 16 selects the time slot in which it isto transmit the data as indicated by a block 114, the RFID tag 16selects the frequency at in which it is to transmit the data asindicated by a block 116, and the RFID tag 16 causes the frequency agileRF transmitter 32 to transmit the data in the selected time slot usingthe selected frequency as indicated by a block 118.

The time at which the RFID tag 16 is to transmit data (the block 106)depends on the state of the long range reader 12. If the reader state ascontained in the header and stored by the RFID tag 16 (block 104)indicates that the long range reader 12 is in the beacon mode, the RFIDtag 16 self-originates the transmission of data. In this state, the RFIDtag 16, for example, may be arranged to transmit data periodically basedon a timer. When the timer indicates that it is time to transmit, theRFID tag 16 starts at the block 100 so that the direct sequence spreadspectrum RF receiver 34 can determine the frequency at which thefrequency agile RF transmitter 32 is to transmit and processingcontinues through the remainder of the blocks shown in FIGS. 9 and 10.In the case where the RFID tag 16 is in the beacon mode, the block 110may be a simple pass through to the block 112.

If the long range reader 12 is an active communication state, the RFIDtag 16 at the block 110 determines whether an interrogation signal forthe RFID tag 16 has been received.

Other reader states for the long range reader 12 are also possible.

As indicated above, the time slot in which the RFID tag 16 transmitsdata may be selected based on the pseudo-randomly generated number. Thefrequency at which the RFID tag 16 transmits data is selected based onthe current position in the hop sequence as parsed (block 106) andstored (block 108) by the RFID tag 16.

The protocol as described above facilitates a long duty cycle and/orinfrequent interrogations by the long range reader 12 because the RFIDtag 16 can quickly receive the necessary information (such as hoppingfrequency) to permit it to transmit data.

As shown in FIG. 11, when it is time for the RFID tag 16 to transmitdata to the short range reader 14 (or portal) as indicated by a block130, the second transceiver 38 of the RFID tag 16 transmits theappropriate information as indicated by a block 132.

The time at which the RFID tag 16 transmits data to the short rangereader 14 (or portal) and the data to be transmitted may be inaccordance with any of the embodiments and/or alternatives describedabove. For example, if the RFID tag 16 receives a signal from the shortrange reader 14 requesting information stored in the memory 40, it istime to transmit, and the second transceiver 38 transmits the requestedinformation to the short range reader 14. As another example, when theRFID tag 16 is being used as a security device, the RFID tag 16 respondsto an interrogation signal from the short range reader 14 or a portal(time to transmit) by sending a signal to the short range reader 14,portal, or long range reader 12 indicating that the RFID tag 16 iswithin the range of the short range reader 14 or portal.

Certain modifications of the present invention have been disclosedabove. Other modifications will occur to those practicing in the art ofthe present invention. For example, the functions of the long rangereader 12 as described above have been confined to reading informationfrom the RFID tag 16. However, the long range reader 12 can also bearranged to write information to the RFID tag 16.

Also, as described above, the long range reader 12 is arranged to readthe tag ID of the RFID tag 16, and the short range reader 14 is arrangedto read other information from the RFID tag 16. However, the long rangereader 12 may be arranged instead to read any combination of tag ID andother information from the RFID tag 16, and the short range reader 14may be similarly arranged to read any combination of the tag ID andother information from the RFID tag 16.

Moreover, although the RFID tag 16 is shown as a microprocessor basedtag in FIG. 2, the RFID tag 16 may instead comprise one or more digitalcircuit elements, and/or a programmable logic array, and/or a dedicatedintegrated circuit, etc.

Furthermore, the long range reader 12 as described above has a range ofseveral hundred feet and could have an expected range of approximately500 feet. However, this range could be longer or shorter depending onthe application and/or other factors. Similarly, the range given abovefor the short range reader 14 could be other than as described above.

Additionally, the transmitter of the first transceiver 30 of the RFIDtag 16 is described above as the frequency agile RF transmitter 32, andthe receiver of the first transceiver 30 of the RFID tag 16 is describedabove as the direct sequence spread spectrum RF receiver 34. However,the RFID tag 16 may instead advantageously use other types oftransmitters and receivers.

Also, the frequency agile RF transmitter 32 and the direct sequencespread spectrum RF receiver 34 are duty cycled as described to conservethe energy of the battery 42. Additionally, the microprocessor 36 mayalso power itself down during the OFF portion of the duty cycle of theRFID tag 16 requiring only the power necessary to determine the ONportion of the duty cycle.

Moreover, as described above, one or more elements of the RFID tag 16,such as the direct sequence spread spectrum RF receiver 34, may be dutycycled. The duty cycles can have equal or unequal ON times and/or equalor unequal OFF times.

Furthermore, the switches 44 and 46 described above may beelectromechanical switches, semiconductor switches, logic elements, etc.

Accordingly, the description of the present invention is to be construedas illustrative only and is for the purpose of teaching those skilled inthe art the best mode of carrying out the invention. The details may bevaried substantially without departing from the spirit of the invention,and the exclusive use of all modifications which are within the scope ofthe appended claims is reserved.

We claim:
 1. A method of communicating information between an RFID tagand first and second readers, the method comprising: controlling a firsttransceiver of the RFID tag so that the first transceiver communicateswith the first reader and so that the first transceiver hassubstantially longer periods during which the first transceiver is notin communication with the first reader than when the first transceiveris in communication with the first reader; and, controlling a secondtransceiver of the RFID tag so that the second transceiver communicateswith the second reader at least during the periods when the firsttransceiver is not in communication with the first reader.
 2. The methodof claim 1 wherein the controlling of the first transceiver of the RFIDtag so that the first transceiver has substantially longer periodsduring which the first transceiver is not in communication with a firstreader than when the first transceiver is in communication with thefirst reader comprises receiving infrequent inquiries from the firstreader.
 3. The method of claim 1 wherein the controlling of the firsttransceiver of the RFID tag so that the first transceiver hassubstantially longer periods during which the first transceiver is notin communication with a first reader than when the first transceiver isin communication with the first reader comprises cycling the firsttransceiver on and off from internal circuitry of the RFID tag.
 4. Themethod of claim 3 wherein the cycling of the first transceiver comprisesduty cycling the first transceiver.
 5. The method of claim 1 wherein thefirst transceiver comprises a transmitter and a receiver, and whereinthe controlling of a first transceiver comprises controlling thereceiver so that the receiver has substantially longer periods duringwhich the receiver is not receiving communications from the first readerthan when the receiver is receiving communications from the firstreader.
 6. The method of claim 5 wherein the receiver comprises a directsequence spread spectrum RF receiver, and wherein the transmittercomprises a frequency agile RF transmitter.
 7. The method of claim 5wherein the controlling of the receiver comprises duty cycling thereceiver.
 8. The method of claim 1 wherein the first transceivercomprises a transmitter and a receiver, wherein the controlling of afirst transceiver comprises controlling the receiver and the transmitterso that the receiver and the transmitter have substantially longerperiods during which the receiver and transmitter are not receiving andtransmitting communications from and to the first reader than when thereceiver and transmitter are receiving and transmitting communicationsfrom and to the first reader.
 9. The method of claim 8 wherein thereceiver comprises a direct sequence spread spectrum RF receiver, andwherein the transmitter comprises a frequency agile RF transmitter. 10.The method of claim 8 wherein the controlling of the receiver and thetransmitter comprises duty cycling the receiver and the transmitter. 11.The method of claim 1 wherein the first transceiver comprises a directsequence spread spectrum RE receiver and a frequency agile RFtransmitter.
 12. The method of claim 1 wherein at least somecommunications between the RFID tag and the first reader are conductedin message frames, wherein each of the message frames comprises a headerand a time slot, wherein the header is transmitted by the first readerand contains a frequency, wherein the time slot comprises a headerportion and a data portion, wherein the header portion is transmitted bythe first reader and also contains the frequency, and wherein thecontrolling of a first transceiver comprises controlling the firsttransceiver so as to transmit data from the RFID tag to the first readerin the data portion of the time slot at the frequency.
 13. The method ofclaim 12 wherein the controlling of the first transceiver so as totransmit data from the RFID tag to the first reader comprisespseudorandomly selecting the time slot.
 14. The method of claim 1wherein at least some communications between the RFID tag and the firstreader are conducted in message frames, wherein each of the messageframes comprises a header and a time slot, wherein the header istransmitted by the first reader and contains a frequency, and whereinthe controlling of a first transceiver comprises controlling the firsttransceiver so as to transmit data from the RFID tag to the first readerin the time slot at the frequency.
 15. The method of claim 14 whereinthe controlling of the first transceiver so as to transmit data from theRFID tag to the first reader comprises pseudorandomly selecting the timeslot.
 16. The method of claim 1 wherein at least some communicationsbetween the RFID tag and the first reader are conducted in messageframes, wherein each of the message frames comprises a header and a timeslot, wherein the header is transmitted by the first reader and containsa hop sequence and a frequency representing a current position in thehop sequence, wherein the time slot comprises a header portion and adata portion, wherein the header portion is transmitted by the firstreader and also contains the hop sequence and the frequency representinga current position in the hop sequence, and wherein the controlling of afirst transceiver comprises controlling the first transceiver so as totransmit data from the RFID tag to the first reader in the data portionof the time slot at the frequency.
 17. The method of claim 16 whereinthe controlling of the first transceiver so as to transmit data from theRFID tag to the first reader comprises pseudorandomly selecting the timeslot.
 18. The method of claim 1 wherein at least some communicationsbetween the RFID tag and the first reader are conducted in messageframes, wherein each of the message frames comprises a header and a timeslot, wherein the header is transmitted by the first reader and containsa hop sequence and a frequency representing a current position in thehop sequence, and wherein the controlling of a first transceivercomprises controlling the first transceiver so as to transmit data fromthe RFID tag to the first reader in the time slot at the frequency. 19.The method of claim 18 wherein the controlling of the first transceiverso as to transmit data from the RFID tag to the first reader comprisespseudorandomly selecting the time slot.
 20. The method of claim 1wherein the controlling of a first transceiver comprises transmittingdata in a time slot pseudorandomly selected by the RFID tag.
 21. Themethod of claim 1 further comprising receiving a reader state from thefirst reader.
 22. The method of claim 21 wherein the reader stateindicates that the RFID tag is to operate in a beacon mode.
 23. Themethod of claim 21 wherein the reader state indicates that the RFID tagis to operate in an active communication mode.
 24. The method of claim 1wherein the controlling of a second transceiver comprises controllingthe second transceiver of the RFID tag so that the second transceivercommunicates with the second reader substantially immediately uponinquiry from the second reader.
 25. A method of communicatinginformation between an RFID tag and first and second readers, the methodcomprising: controlling a first transceiver of the RFID tag so that thefirst transceiver communicates with the first reader and so that thefirst transceiver has substantially longer periods during which thefirst transceiver cannot communicate with the first reader than when thefirst transceiver can communicate with the first reader; and,controlling a second transceiver of the RFID tag so that the secondtransceiver communicates with the second reader at least during theperiods when the first transceiver cannot communicate with the firstreader.
 26. The method of claim 25 wherein the controlling of the firsttransceiver comprises duty cycling the first transceiver on and off frominternal circuitry of the RFID tag.
 27. The method of claim 25 whereinthe first transceiver comprises a direct sequence spread spectrum RFreceiver and a frequency agile RF transmitter.
 28. The method of claim25 wherein at least some communications between the RFID tag and thefirst reader are conducted in message frames, wherein each of themessage frames comprises a header and a time slot, wherein the header istransmitted by the first reader and contains a frequency, wherein thetime slot comprises a header portion and a data portion, wherein theheader portion is transmitted by the first reader and also contains thefrequency, and wherein the controlling of a first transceiver comprisescontrolling the first transceiver so as to transmit data from the RFIDtag to the first reader in the data portion of the time slot at thefrequency.
 29. The method of claim 28 wherein the controlling of thefirst transceiver comprises pseudorandomly selecting the time slot. 30.The method of claim 25 wherein at least some communications between theRFID tag and the first reader are conducted in message frames, whereineach of the message frames comprises a header and a time slot, whereinthe header is transmitted by the first reader and contains a frequency,and wherein the controlling of a first transceiver comprises controllingthe first transceiver so as to transmit data from the RFID tag to thefirst reader in the time slot at the frequency.
 31. The method of claim30 wherein the controlling of the first transceiver comprisespseudorandomly selecting the time slot.
 32. The method of claim 25wherein at least some communications between the RFID tag and the firstreader are conducted in message frames, wherein each of the messageframes comprises a header and a time slot, wherein the header istransmitted by the first reader and contains a hop sequence and afrequency representing a current position in the hop sequence, whereinthe time slot comprises a header portion and a data portion, wherein theheader portion is transmitted by the first reader and also contains thehop sequence and the frequency representing a current position in thehop sequence, and wherein the controlling of a first transceivercomprises controlling the first transceiver so as to transmit data fromthe RFID tag to the first reader in the data portion of the time slot atthe frequency.
 33. The method of claim 32 wherein the controlling of thefirst transceiver comprises pseudorandomly selecting the time slot. 34.The method of claim 25 wherein at least some communications between theRFID tag and the first reader are conducted in message frames, whereineach of the message frames comprises a header and a time slot, whereinthe header is transmitted by the first reader and contains a hopsequence and a frequency representing a current position in the hopsequence, and wherein the controlling of a first transceiver comprisescontrolling the first transceiver so as to transmit data from the RFIDtag to the first reader in the time slot at the frequency.
 35. Themethod of claim 34 wherein the controlling of the first transceivercomprises pseudorandomly selecting the time slot.
 36. A method ofcommunicating information between an RFID tag and first and secondreaders, the method comprising: controlling a first transceiver of theRFID tag so that the first transceiver communicates with the firstreader and so that the first transceiver has substantially longerperiods during which the first transceiver is not in communication withthe first reader than when the first transceiver is in communicationwith the first reader, wherein at least some communications between theRFID tag and the first reader are conducted in message frames, whereineach of the message frames comprises a header and a time slot, whereinthe header is transmitted by the first reader, and wherein the firsttransceiver is controlled so as to transmit data in the time slot; and,controlling a second transceiver of the RFID tag so that the secondtransceiver communicates with the second reader at least during theperiods when the first transceiver is not in communication with thefirst reader.
 37. The method of claim 36 wherein the controlling of thefirst transceiver of the RFID tag comprises receiving infrequentinquiries from the first reader.
 38. The method of claim 36 wherein thecontrolling of the first transceiver of the RFID tag comprises dutycycling the first transceiver on and off from internal circuitry of theRFID tag.
 39. The method of claim 36 wherein the first transceivercomprises a direct sequence spread spectrum RF receiver and a frequencyagile RF transmitter.
 40. The method of claim 36 wherein the header ofthe message frames contains a frequency, wherein the time slot comprisesa header portion and a data portion, and wherein the header portion istransmitted by the first reader and also contains the frequency.
 41. Themethod of claim 40 wherein the controlling of the first transceivercomprises pseudorandomly selecting the time slot.
 42. The method ofclaim 36 wherein the header of the message frames contains a frequency,and wherein the controlling of a first transceiver comprises controllingthe first transceiver so as to transmit data from the RFID tag to thefirst reader at the frequency.
 43. The method of claim 42 wherein thecontrolling of the first transceiver comprises pseudorandomly selectingthe time slot.
 44. The method of claim 36 wherein the header of themessage frames contains a hop sequence and a frequency representing acurrent position in the hop sequence, wherein the time slot comprises aheader portion and a data portion, wherein the header portion istransmitted by the first reader and also contains the hop sequence andthe frequency representing a current position in the hop sequence, andwherein the controlling of a first transceiver comprises controlling thefirst transceiver so as to transmit data from the RFID tag to the firstreader in the data portion of the time slot at the frequency.
 45. Themethod of claim 44 wherein the controlling of the first transceivercomprises pseudorandomly selecting the time slot.
 46. The method ofclaim 36 wherein the header of the message frames is transmitted by thefirst reader and contains a hop sequence and a frequency representing acurrent position in the hop sequence, and wherein the controlling of afirst transceiver comprises controlling the first transceiver so as totransmit data from the RFID tag to the first reader in the time slot atthe frequency.
 47. The method of claim 46 wherein the controlling of thefirst transceiver comprises pseudorandomly selecting the time slot. 48.The method of claim 36 further comprising pseudorandomly selecting thetime slot in which the first transceiver transmits data.
 49. The methodof claim 36 wherein the header contains a reader state from the firstreader.
 50. The method of claim 49 wherein the reader state indicatesthat the RFID tag is to operate in a beacon mode.
 51. The method ofclaim 49 wherein the reader state indicates that the RFID tag is tooperate in an active communication mode.